Composite wire



Aug. 23, 1966 M. w. HAGADORN ETAL 3,268,305

COMPOSITE WIRE Filed Aug. 17, 1964 INVENTORS Mmow Hf HflflDaRNl BYSAMUEL JMaA/moMsRY ATTORNEY United States Patent I Patented August 23, 1966 3,268,305 coMPosrTE WIRE Mai-old W. Hagadcrn and Samuel J. Montgomery, Emporium, Pa, assignors to SylvaniaElectric Products Inc., a corporation of Delaware Filed Aug. 17, 1964, Ser. No. 389,992 11 Claims. (Cl. 29-1835) .charge device.

For many years such devices as electrical lamps and electron discharge devices have employed refractory met- ..als and especially tungsten in the heaters or cathodes thereof because of such unique properties asa high melt- .ing temperature, high tensile strength, and a high modulus of elasticity. Also, it is and has been a common practice to dope the refractory metal in order to prevent the metal from sagging during the high temperature operational use thereof. For example, a double additive of sodium or potassium mixed with anonvolatile substance such as silicon, aluminum, or thorium oxide in an amount of up to about 1.0% by weight is frequently used as a doping agent in tungsten wire employeddn devices operated above the recrystallization temperature of the tungsten.

However, it is well known that doped refractory metals lose their ductility after operational use at a temperature above the recrystallization temperature of the refractory metal. For example, it has been found that a doped tungsten .wire coated .Withaninsulating layer of aluminum oxide and used as a heater in an indirectly heated electron discharge device is very brittleafter operational use at a temperature of up to about '1600" C. As a result, such heaters are most susceptible to shock and vibration which causes breakage-of the brittle wire and renders the discharge device useless.

As a solution to this problem of brittleness after recrystallization, rh'enium-tungsten alloys have been developed which usually includerhenium in the range of about 3.2 to 5.0% by weight. For example,apreferred alloy is doped tungsten which includes rhenium in an amount of about-3.26% by weight of the tungsten. However, even though rhenium-tungsten alloys have proven to be superior to the previously-known doped refractory metal materials, it has been found that fabricating such alloys requires additional swaging, drawing, and annealing operations which not only reduce the rateofproduction but, more importantly, increase'the cost of the final product. Further, rhenium is an expensive material in comparison with tungsten, and it'has been found that tungsten-rhenium alloys with about 3.2% or less by Weight of rhenium therein do not have the desired ductility after having been operated above the recrystallization temperature of the alloy. Thus,'a satisfactory structure requires a rhenium-tungsten alloy containing morethan 3.0% rhenium and the. cost of such a structure greatly exceeds the cost of prior known refractory met-a1 structures.

Additionally, it has been found that a heater for an indirectly heated electron discharge device, formed from a base metal such as the previously described tungsten and rhenium-tungsten alloys and covered with a layer of insulating material, has a tendency to decrease in resistivity during the operational use thereof. Although the exact cause of'this change in resistivity is not known.

it is believed that the decrease therein is attributable to the heater of an indirectly heated electron discharge device, which has been found to be particularly prevalent in tungsten and rhenium-tungsten alloys, causes an increase in the heater currentof the device during the operational use thereof, and an increase in heater current has a deleterious effect upon the useful operational period of-the discharge device.

Therefore, it is an :object of this invention-to provide a composite structure having ductility after operational use above the recrystallizationtemperature thereof.

A further object of the invention is to reduce the cost offabricating a composite structure having ductility after operational use above the recrystallization temperature thereof.

A still further object of the invention is to'enhance the electrical stability of a structure suitable for operational use above the recrystallization temperatu'rethereof.

Another object of the invention is to provide a heater for an'electron discharge devicehaving a stable resistivity during operational use above the recrystallization temperature thereof and ductility after such operational use. Still another object of the invention is to reduce the fabrication cost of an improved heater suitable for use tion temperature thereof.

These and'other objects are achieved in one aspect of the invention by a composite structure which includes a refractory metal base having a layer of rheniumin the range of about 0.1 to 3.0% by weight of the base surrounding and attached thereto.

For a better understanding of the present invention, together with other and further objects, advantages, and capabilities thereof, reference is made to the following disclosure in connection with the accompanying drawings in which:

FIG. 1 is a cross-sectional view illustrating one'aspect of the relationship of the base and metal layer of the composite structure;

FIG. 2 is an elevational view of a heater applicable for use in an indirectly heated cathode assembly for an electron discharge device and employing one embodiment of the composite structure of FIG. 1; and

FIG. 3 is an elevational view of a directly heated electron discharge device employing another embodiment of the composite structure of FIG. 1.

Referring to the drawings, FIG. 1 illustrates a composite structure 5 which includes a metal base 7 and an attached metal layer 9. The base 7 is preferably of tungsten or an alloy thereof although any of the refractory metals and refractory metal alloys may be used. For example, metals and alloys from the refractory metal group, which includes molybdenum, chromium, vanadium, titanium, zirconium, tantalum, columbium, rhodium, ruthenium, and palladium, are equally applicable and appropriate materials for the base 7.

Surrounding and attached to the base 7 is a metal layer 9 of rhenium. This metal layer 9 is attached by any one of a number of ordinary processes for afiixing a metal layer 9 to a metal base 7. As an example, an ingot of sintered and compacted tungsten doped with .a total of approximately 1.0% by weight of calcium chloride and oxides of sodium, potassium, aluminum, and silicon is swaged to a diameter of about 0.015 inch. Then a layer of rhenium is plated onto the 0.015 inch tungsten and the resultant rhenium-plated tungsten is drawn to a diameter of about 0.00625 inch or, in a more common method of designation, about 75 milligrams per 200 millimeter length. As a result, there is provide a composite structure which includes a tungsten base 7 having a rhenium metal layer 9 thereon with the layer 9 having a uniform thickness and a weight of approximately 0.7% of the base 7.

As mentioned above and illustrated in FIG. 2, the composite structure 5 of FIG. 1 may be employed as a heater 11 in an indirectly heated electron discharge device. The heater 11 includes the composite metal structure 5 and a layer 13 of insulating material surrounding and adhered thereto. As is norm-a1 for indirectly heated electron discharge devices, the layer 13 of insulating material is preferably alumina mixed with a volatile binder. Also, the heater 11 has leg portions 15 for convenient electrical attachmentwhich either have not been covered with the layer 13 of insulating material or wherefrom the layer 13 of insulating material has been moved, depending upon the fabrication technique employed.

Also, the composite metal structure 5 is equally applicable as a cathode or electron source in a directly heated electron discharge device 17 :as illustrated in FIG. 3. Therein, the discharge device 17 includes the usual glass stem support 19 having a plurality of electrical conductors 21 sealed therein and extending therethrough, an anode electrode 23 supported intermediate a pair of insulator spacers 25 and 27 upheld by the conductors 21, a grid electrode 29 within the anode electrode 23 and attached to certain ones of the conductors 21, and a pair of directly heated cathodes 31 and 33.

Each one of the cathodes 31 and 33 includes a bottom tab 35 attached to one of the conductors 21 and a top tab 37 welded to a tension spring 39 attached to a tension adjustment support 41. Intermediate and attached to the bottom and top tabs 35 and 37 is the composite structure 5 with a layer 43 of potentially emissive materials thereon and adhered thereto. The layer 43 of potentially emissive material is preferably the usual mixture of alkaline earth carbonates suspended in a binder, although numerous other potentially emissive materials are equally applicable and appropriate.

As a specific example of one application of the composite structure 5 of FIG. 1, a doped tungsten ingot was prepared and plated with a layer 9 of rhenium as previously described and drawn into wire having a weight of about 75 mg./ 200 mm., with the rhenium comprising approximately 0.7% by weight of the base 7. This structure 5 was then coated with a layer of insulating material and cut and formed into heaters 11 as illustrated in FIG. 2. Thereafter, the heaters 11 were inserted into indirectly heated electron discharge devices and processed in a manner normally used for such devices.

In a similar manner, regular doped tungsten and a rhenium-tungsten alloy containing approximately 3.26% by weight of rhenium were fabricated into heaters, inserted into similar discharge devices, and processed.

Total Material Percent Bends Rhenium Before Breakage Doped Tungsten 0. 0 1 Rhenium-coated Tungsten" 0. 7 320 Rhenium-tungsten Alloy 3. 26 300 As can be readily observed, the base material 7, having a metal layer 9 of rhenium thereon, exhibits improved ductility over either regular tungsten or a rhenium-tungsten alloy after operation use above the recrystallization temperature of the material. Further, the percentage of rhenium required to provide such results has been greatly decreased resulting in a reduction in material cost.

In an example of the resistive stability obtainable under adverse operational conditions, a test was prepared wherein doped tungsten in the range of about 5.02 mg./200 mm. had attached thereto a layer of rhenium comprising about 2.5% by weight of the base material. In the manner described above, heaters for an indirectly heated electron discharge device were prepared, inserted, and the discharge devices processed in a normal manner. Similarly, regular doped tungsten and a rhenium-tungsten alloy containing about 3.26% by weight of rhenium were prepared and processed. Following, the discharge devices normal- 1y designed for operation with about 6.3 volts applied to the heater were subjected to a heater voltage in excess of the above-designed rating for an extended period of time and provided the following result:

Again, as can be readily observed, the composite structure, which included a doped tungsten base 7 having a rhenium layer 9 of about 2.5% by weight of the base thereon, had the smallest percentage change in heater current and hence in resistivity of any of the materials compared therewith. More-over, this advantageous position exists for each of the various conditions of voltage applied to the heater of the discharge device.

Although it is known that rhenium promotes a redistribution of oxygen which is an embrittling impurity in refractory metals and also does not form a carbide but rather does readily diffuse carbon therethrough, an exact explanation for the unique and unexpected results obtainable from a composite structure, which includes a refractory met-a1 coated with a rhenium layer, is not presently available. However, it has been found that a rhenium coated refractory metal does retain a ductility after operational use above the recrystallization temperature thereof, which is unobtainable in any other known type of composite structure or alloy.

Also, the structure has a resistance stability, weldability, tensile strength, and resistance to water vapor which is believed to be unavailable in any of the known types of composite structures. Moreover, the fabrication cost as well as the material cost of the above-described composite structure have been found to be much less than the cost of any known structure which even approaches the desirable and advantageous characteristics provided thereby.

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention as defined by the appended claims.

What is claimed is:

1. A composite wire structure having ductility after operation above the recrystallization temperature thereof and comprising a base selected from the alloys and metals of the refractory metal group and a layer of rhenium surrounding and attached to said base, said layer being of substantially uniform thickness and in the range of about 0.1 to 3.0% by weight of said base.

2. A composite wire structure having ductility after operation above the recrystallization temperature thereof and comprising a base and an outer surrounding layer attached to the base, said base being selected from metals and alloys of the metals of the group consisting of tungsten, molybdenum, chromium, vanadium, titanium, zirconium, tantalum, columbium, rhodium, ruthenium, and palladium, and said layer being rhenium in the range of about 0.1 to 3.0% by weight of said base.

3. A wire having ductility after operation above the recrystallization temperature thereof and comprising a base selected from the alloys and metals of the refractory metal group and including up to about 1.0% by weight of doping agents and a layer of metal surroundings and attached to said base, said'layer of metal being rhenium of substantially uniform thickness and in the range of about 0:1 to 3.0% by weight of said base. 7

4. A wire having ductility after operation above the recrystallization temperature thereof and comprising a base selected from the alloys and metals of the refractory metal group and including up to about 1. 0% by weight of doping agents, said agents being selected from th group which includes sodium oxide, potassium oxide, calcium chloride, aluminum oxide, and silicon oxide, and a layer surrounding and attached to said base, said layer being rhenium having a substantially uniform thickness and in the range of about 0J1 to 3.0% by weight of said base.

5. A wire having ductility and a more stable resistance thereof after operation at a temperature of up to about 1600 C. and comprising a base of tungsten metal doped with up to about 1. 0% by weight of an oxide selected from the group consisting of sodium and potassium and a nonvolatile substance from the group consisting of calcium chloride, aluminum oxide, and silicon oxide, and a substantially uniformly thick metal layer surrounding and attached to said base, said layer being rhenium in the range of about 0.1 to 3.0% by weight of said base.

6. A heater for an indirectly heated electron discharge device comprising a refractory metal base, a layer of rhenium metal surrounding and attached to said base, and a layer of insulating coating surrounding and adhered to said layer of rhenium, said rhenium layer being of substantially uniform thickness and in the range of about 0.1 to 3.0% by Weight of said base and said heater having ductility after operation above the recrystallization temperature of said base and a stable resistance during'said operation.

7. A heater for an indirectly heated electron discharge device comprising a metal base, a metal layer of substantially uniform thickness surrounding and attached to said base, and a layer of insulating material surrounding and adhered to said metal layer, said base being of the metals and alloys of the metals selected from the group consisting of tungsten, molybdenum, chromium, vanadium, titanium, zirconium, tantalum, columbium, rhodium, ruthenium, and palladium, and said metal layer being rhenium in an amount of about 0.1 to 3.0% by weight of said base whereby said heater has a stable resistive value during operational use thereof and is ductile after operational use above the recrystallization temperature of said base.

8. A heater for an indirectly heated electron discharge device comprising a metal base, a metal layer of substantially uniform thickness surrounding and attached to said base, and a layer of insulating material surrounding and adhered to said metal layer, said base being selected from the alloys and metals of the refractory metal group and including up to about 1.0% by weight of doping agents,

said metal layer being rhenium in an amount of about 0.1 to 3.0% by weight of said base, and said insulating layer consisting essentially of aluminum oxide.

9. A heater for an indirectly heated electron discharge device comprising a tungsten base, a metal layer of rhenium having a substantially uniform thickness and in an amount of about 0.1 to 1.0% by weight of said base,

said metal layer surrounding and attached to said base, and an insulating layer consisting essentially of aluminum oxide surrounding and adhered to said metal layer, said heater having ductility after operation above a temperature of up to about 1600 C.

10. A cathode for an electron discharge device comprising a refractory metal base, a rhenium layer attached to said base, said layer having a uniform thickness and in an amount of about 0.1 to 3.0% by weight of said base, and a layer of potentially emissive materials adhered to and covering said rhenium layer, said cathode having a stable resistivity during operation above the recrystallization temperature of said base and ductility after said operation above said recrystallization temperature.

11. A cathode for an electron discharge device comprising a metal base, a metal layer attached to said base, and a layer of potentially emissive materials adhered to said metal layer, said base being selected from the metals and alloys of the group consisting of tungsten, molybdenum, chromium, vanadium, titanium, zirconium, tantalum, columbium, rhodium, ruthenium, and palladium, and including doping agents up to about 1.0% by weight of said base, and said metal layer being rhenium in an amount of about 0.1 to 3.0% by weight of said base and of substantially uniform thickness, said cathode having ductility after operation above the recrystallization temperature of said base.

References Cited by the Examiner UNITED STATES PATENTS 2,914,640 11/ 1959 Grattidge 29-198 XR 3,024,522 5/1962 Caccioti 29--198 XR HYLAND BIZOT, Primary Examiner. 

2. A COMPOSITE WIRE STRUCTURE HAVING DUCTILITY AFTER OPERATION ABOVE THE RECRYSTALLIZATION TEMPERATURE THEREOF AND COMPRISING A BASE AND AN OUTER SURROUNDING LAYER ATTACHED TO THE BASE, SAID BASE BING SELECTED FROM METALS AND ALLOYS OF THE METALS OF THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, CHROMIUM, VANADIUM, TITANIUM, ZIRCONIUM, TANTALUM, COLUMBIUM, RHODIUM, RUTHENIUM, AND PALLADIUM, AND SAID LAYER BEING RHENIUM IN THE RANGE OF ABOUT 0.1 TO 3.0% BY WEIGHT OF SAID BASE. 